1. Field of the Invention
The present invention relates to an apparatus for the electromagnetic forming of materials as well as a method for manufacturing this apparatus.
2. Description of Related Art
Electromagnetic (EM) forming uses the pressure created by a pulsed electromagnetic field in combination with traditional sheet forming technologies on conventional presses to shape materials. An electromagnetic force is defined as a force developed by the passage of an electrical current. EM forming is typically accomplished by the use of an electric current source, a multi-turn solenoid coil and a die. The electrical current leaves from the source at one end of the coil and travels through the coil to the other end. During the high-voltage discharge of capacitors through the coil, a strong electromagnetic field is generated which induces eddy current in the workpiece. The interaction of electromagnetic fields generated by the direct current in the coil and the induced current in the workpiece results in high intensity repelling force, which accelerates the workpiece into the die cavity.
Today, there are two prevalent ways of forming materials using electromagnetic principles. In the more popular method, a shaper generates a secondary electromagnetic field around itself. This electromagnetic field induces the secondary eddy current in the workpiece. As a result of the interaction of the electromagnetic fields, the workpiece repels from the shaper and accelerates toward the corners of a lower die driven by electromagnetic pressure. In another method, the pressure generated by the EM field of the coil acts directly on the workpiece, forcing it against the die.
While electromagnetic forming applications have advantages over conventional forming techniques, including conformance within tighter design dimensions and reducing residual stresses, they also have disadvantages. EM forming applications are limited to production at low volumes since the coils quickly deform due to their low material strength and overheating. Moreover, the workpiece still holds a significant amount of residual stresses that cause it to spring back towards its initial shape. Also, EM forming application can require a substantial amount of electricity and the coils can take a significant amount of time to machine using traditional cutting methods such as end milling. Alternatively, the coil can be formed by winding material into the desired shape; however, this type of coil formation typically results in a less stiff coil having strong residual stresses.
With electromagnetic forming, the coil can be subjected to high stresses during repetitive operations, thus causing the coil to deform. U.S. Pat. No. 3,704,506 suggests using a supportive coil casing to resist the coil's tendency to deform. The use of a casing around the coil is popular but not very effective in increasing the cycle life of the coil. Similarly, U.S. Pat. No. 6,128,935 uses tie rods extending through the coils to resist movement of the coil. However, this arrangement does not provide the coil with enough support as the rods do not extend through the coil and coil casing. Moreover, if the rods are made of conductive material, the coil may short circuit. Therefore, there exists a need to provide adequate reinforcement to the coil permitting higher rates of production.
Moreover, with electromagnetic forming, high temperatures can be generated, thus necessitating a need for cooling the coil. Other designs have attempted to overcome this shortcoming with the use of a cooling agent. U.S. Pat. No. 3,842,630 suggests a method of cooling an EM forming apparatus by routing a cooling agent through a chamber underneath the workpiece. This approach does not actively cool the tool as the working area of the coil is not in direct contact with the coolant. Likewise, U.S. Pat. No. 5,113,736 fails to actively cool the tool as it suggests using a fan that blows air into a cooling housing mounted to the top of the coil. U.S. Pat. No. 3,195,335 discloses pumping coolant through the conductor. This requires the use of a hollowed coil that will have a significantly lower material strength than a filled coil. Moreover, using supportive rods with this coil design is less feasible as the coolant is more likely to leak out of the apertures for the supportive rods. Therefore, there further exists a need to actively cool the tool permitting higher rates of production without overheating.
Residual stresses in materials after forming cause them to spring back to their initial shape. U.S. Patent Application 2003/0182005 A1 attempts to solve this problem by determining a die profile for forming a metal part that will reduce material spring back. However, this method limits the possible shapes that the material can undertake. Therefore, there further exists a need to reduce residual stresses in formed material to prevent spring back.